This way it won't be necessary to update the constructor line each time Visual Studio rewrites the *.Designer.cs file.
I know it is possible to programmatically add a GLControl to a form, but AFAIK you loose the comfort of the Designer that way.

Such a mechanism would help a lot in case of complicated GUIs with two or more OpenGL 3+ GLControls. Currently, I'm using about six of them and maintenance is becoming a nightmare.